Apparatus and techniques for generating bunched ion beam

ABSTRACT

An apparatus may include a first grounded drift tube, arranged to accept a continuous ion beam, at least two AC drift tubes, arranged in series, downstream to the first grounded drift tube, and a second grounded drift tube, downstream to the at least two AC drift tubes. The apparatus may include an AC voltage assembly, electrically coupled to at least two AC drift tubes. The AC voltage assembly may include a first AC voltage source, coupled to deliver a first AC voltage signal at a first frequency to a first AC drift tube of at least two AC drift tubes. The AC voltage assembly may further include a second AC voltage source, coupled to deliver a second AC voltage signal at a second frequency to a second AC drift tube of the at least two AC drift tubes, wherein the second frequency comprises an integral multiple of the first frequency.

FIELD OF THE DISCLOSURE

The disclosure relates generally to ion implantation apparatus and more particularly to high energy beamline ion implanters.

BACKGROUND OF THE DISCLOSURE

Ion implantation is a process of introducing dopants or impurities into a substrate via bombardment. Ion implantation systems may comprise an ion source and a series of beam-line components. The ion source may comprise a chamber where ions are generated. The beam-line components, may include, for example, a mass analyzer, a collimator, and various components to accelerate or decelerate the ion beam. Much like a series of optical lenses for manipulating a light beam, the beam-line components can filter, focus, and manipulate an ion beam having particular species, shape, energy, and/or other qualities. The ion beam passes through the beam-line components and may be directed toward a substrate mounted on a platen or clamp.

One type of ion implanter suitable for generating ion beams of medium energy and high energy uses a linear accelerator, or LINAC, where a series of electrodes arranged as tubes around the beam accelerate the ion beam to increasingly higher energy along the succession of tubes. The various electrodes may be arranged in a series of stages where a given electrode in a given stage receives an AC voltage signal to accelerate the ion beam.

LINACs employ initial stages that bunch an ion beam as the beam is conducted through the beamline. An initial stage of a LINAC may be referred to as a buncher, where a continuous ion beam is received by the buncher and is output as a bunched ion beam in packets. Depending upon the frequency of the AC voltage signal and the amplitude, the acceptance or phase capture of an ion beam conducted through a known “double-gap” buncher using one powered electrode may be on the order of 30-35%, meaning that 65% of more of beam current is lost while being conducted into the acceleration stages of the linear accelerator.

With respect to these and other considerations, the present disclosure is provided.

BRIEF SUMMARY

In one embodiment, an apparatus may include a multi-ring drift tube assembly. The multi-ring drift tube assembly may include a first grounded drift tube, arranged to accept a continuous ion beam, at least two AC drift tubes, arranged in series, downstream to the first grounded drift tube, and a second grounded drift tube, downstream to the at least two AC drift tubes. The apparatus may include an AC voltage assembly, electrically coupled to the at least two AC drift tubes. The AC voltage assembly may include a first AC voltage source, coupled to deliver a first AC voltage signal at a first frequency to a first AC drift tube of at least two AC drift tubes. The AC voltage assembly may further include a second AC voltage source, coupled to deliver a second AC voltage signal at a second frequency to a second AC drift tube of the at least two AC drift tubes, wherein the second frequency comprises an integral multiple of the first frequency.

In another embodiment, an ion implantation system may include an ion source to generate a continuous ion beam, and a drift tube assembly, disposed downstream of the ion source. The drift tube assembly may include a first grounded drift tube, arranged to accept the continuous ion beam, an AC drift tube assembly, arranged downstream to the first grounded drift tube, and a second grounded drift tube, downstream to the AC drift tube assembly. The ion implantation system may further include an AC voltage assembly, arranged to send to the AC drift tube assembly a first AC voltage signal at a first frequency, and a second AC voltage signal at a second frequency, wherein the second frequency comprises an integral multiple of the first frequency.

In a further embodiment, a method of treating an ion beam, may include generating the ion beam as a continuous ion beam and receiving the continuous ion beam in a multi-ring drift tube assembly. The method may include conducting the ion beam through a first AC drift tube of the multi-ring drift tube assembly while applying a first AC voltage signal at a first frequency to the first AC drift tube. The method may further include conducting the ion beam through a second AC drift tube of the multi-ring drift tube assembly, downstream of the first AC drift tube while applying a second AC voltage signal at a second frequency to the second AC drift tube. The second frequency may comprise an integral multiple of the first frequency, wherein the ion beam is output from the multi-ring drift tube assembly as a bunched ion beam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an exemplary ion implantation system, according to embodiments of the disclosure;

FIG. 1B shows another ion implantation system, according to embodiments of the disclosure;

FIG. 2 shows an exemplary buncher, according to embodiments of the disclosure;

FIG. 3 shows another exemplary buncher, according to other embodiments of the disclosure;

FIG. 4 depicts results of modeling of operation of a drift tube assembly, according to embodiments of the disclosure;

FIG. 5A and FIG. 5B are graphs illustrating the phase behavior of different rays of ion beams treated by different bunchers, highlighting benefits of the present embodiments; and

FIG. 6 depicts an exemplary process flow according to some embodiments of the disclosure.

The drawings are not necessarily to scale. The drawings are merely representations, not intended to portray specific parameters of the disclosure. The drawings are intended to depict exemplary embodiments of the disclosure, and therefore are not be considered as limiting in scope. In the drawings, like numbering represents like elements.

DETAILED DESCRIPTION

An apparatus, system and method in accordance with the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, where embodiments of the system and method are shown. The system and method may be embodied in many different forms and are not be construed as being limited to the embodiments set forth herein. Instead, these embodiments are provided so this disclosure will be thorough and complete, and will fully convey the scope of the system and method to those skilled in the art.

As used herein, an element or operation recited in the singular and proceeded with the word “a” or “an” are understood as potentially including plural elements or operations as well. Furthermore, references to “one embodiment” of the present disclosure are not intended to be interpreted as precluding the existence of additional embodiments also incorporating the recited features.

Provided herein are approaches for improved high energy ion implantation systems, based upon a beamline architecture. For brevity, an ion implantation system may also be referred to herein as an “ion implanter.” Various embodiments provide novel configurations for providing the capability of generating high energy ions, where the final ion energy delivered to a substrate may be 300 keV, 500 keV, 1 MeV or greater. In exemplary embodiments, a novel buncher design may be employed to process an ion beam in a manner that increases the acceptance of the ion beam, as described below.

Referring now to FIG. 1A, an exemplary ion implanter, shown as implantation system 100, is depicted in block form. The ion implantation system 100 may represent a beamline ion implanter, with some elements omitted for clarity of explanation. The ion implantation system 100 may include an ion source 102, and a gas box 107 held at high voltage as known in the art. The ion source 102 may include extraction components and filters (not shown) to generate an ion beam 106 at a first energy. Examples of suitable ion energy for the first ion energy range from 5 keV to 100 keV, while the embodiments are not limited in this context. To form a high energy ion beam, the ion implantation system 100 includes various additional components for accelerating the ion beam 106.

The ion implantation system 100 may include an analyzer 110, functioning to analyze a received ion beam. Thus, in some embodiments, the analyzer 110 may receive the ion beam 106 with an energy imparted by extraction optics located at the ion source 102, where the ion energy is in the range of 100 keV or below, and in particular, 80 keV or below. In other embodiments, the analyzer 110 may receive the ion beam accelerated by a DC accelerator column to higher energies such as 200 keV, 250 keV, 300 keV, 400 keV, or 500 keV. The embodiments are not limited in this context. The ion implantation system 100 may also include a buncher 130, and a linear accelerator 114 (shown in the dashed line), disposed downstream of the buncher 130. The operation of buncher 130 is detailed below. In brief, the buncher 130 is disposed downstream of the upstream beamline 111, to accept the ion beam 106 as a continuous ion beam (or DC ion beam), and to output the beam as a bunched ion beam. In a bunched ion beam the ion beam is output in discrete packets. At the same time, the energy of the ion beam may be increased by the buncher 130. The linear accelerator 114 may include a plurality of accelerator stages 126, arranged in series, as shown. The accelerator stages 126 may act similarly to the buncher, to output bunched ion beams at a given stage, and to accelerate the ion beam to a higher energy in stages. Thus, a buncher may be considered to be a first accelerator stage, differing from downstream accelerator stages in that the ion beam is received as a continuous ion beam.

In various embodiments, the ion implantation system 100 may include additional components, such as a filter magnet 116, a scanner 118 and collimator 120, where the general functions of the filter magnet 116, scanner 118 and collimator 120 are well known and will not be described herein in further detail. As such, a high energy ion beam, represented by the high energy ion beam 115, after acceleration by the linear accelerator 114, may be delivered to an end station 122 for processing of a substrate 124.

In some embodiments, where the ion beam 106 is provided directly to the analyzer 110, the buncher 130 may receive the ion beam 106 as a continuous ion beam at a relatively lower energy, such as less than 100 keV, as noted. In other embodiments, where the ion implantation system includes a DC accelerator column, the ion beam 106 may be accelerated to be fed as a continuous ion beam at energies of up to 500 keV or greater. In these different cases the exact alternating current (AC) voltages applied by the buncher 130 may be adjusted according to the ion energy of the continuous ion beam received by the buncher 130.

FIG. 1B shows an embodiment of an ion implantation system 100 A, including a DC accelerator column 108, disposed downstream of the ion source 102, and arranged to accelerate the ion beam 106 to produce an accelerated ion beam 109 at a second ion energy, where the second ion energy is greater than the first ion energy, generated by the ion source 102. The DC accelerator column 108 may be arranged as in known DC accelerator columns, such as those columns used in medium energy ion implanters. The DC accelerator column may accelerate the ion beam 106, wherein the accelerated ion beam 109 is received by the analyzer 110 and buncher 130 at an energy such as 200 keV, 250 keV, 300 keV, 400 keV, or 500 keV. Otherwise, the ion implantation system 100A may function similarly to ion implantation system 100.

FIG. 2 shows the structure of an exemplary buncher of a linear accelerator, shown as buncher 130, according to embodiments of the disclosure. The buncher 130 may include a drift tube assembly 150, including a first grounded drift tube 152, arranged to accept a continuous ion beam, shown as accelerated ion beam 109. As shown, the first grounded drift tube 152 is connected to electrical ground. The drift tube assembly 150 may further include an AC drift tube assembly, arranged downstream to the first grounded drift tube 152. As discussed in detail below, the AC drift tube assembly 156 is arranged to receive an AC voltage signal, generally in the radio frequency range (RF range), which signal functions to accelerate and manipulate the accelerated ion beam 109. In the embodiment of FIG. 2, the AC drift tube assembly 156 includes just one AC drift tube. In other embodiments, an AC drift tube assembly 156 may include multiple AC drift tubes.

The drift tube assembly 150 further includes a second grounded drift tube 154, downstream to the AC drift tube assembly 156. As a whole, the drift tube assembly 150 is arranged as hollow cylinders to receive a continuous ion beam, conduct the ion beam through the hollow cylinders, and accelerate the ion beam in a manner that bunches the ion beam into discrete packets, shown as packet 109A, to be received and further accelerated by an acceleration stage 158, located downstream. The drift tube assembly 150 may be constructed of graphite or similar suitable material configured to minimize contamination of an ion beam conducted therethrough. The subsequent accelerating stages, indicated by acceleration stage 158, may operate at a well-defined frequency ω, and the capture of the bunches into this accelerating structure may be limited to approximately ±5° of phase angle with respect to this fundamental angular frequency ω. In order to transmit the largest possible current through the entire beamline, arranging the buncher 130 to produce one bunch for each cycle of this fundamental frequency ω is desirable.

As shown in FIG. 2, the buncher 130 further includes an AC voltage assembly 140, arranged to send to the AC drift tube assembly 156, an AC voltage signal to drive a changing voltage at a powered drift tube of the AC drift tube assembly 156. The varying voltage on the AC drift tube assembly 156 assembly provides different acceleration to the ions, depending on the arrival time of the ions at the AC drift tube assembly 156. In this way, the trailing end 109A1 of the bunch 109A is given more velocity than the leading end 109A2 of the bunch 109A, and the whole of the bunch 109A becomes as compact as possible when arriving at the acceleration stage 158. In various embodiments, the AC voltage signal may be a composite of a plurality of individual AC voltage signals, superimposed to generate an AC voltage signal in a manner to provide improved bunching of a continuous ion beam. In various embodiments, the AC voltage assembly 140 may generate a first AC voltage signal at a first frequency, and a second AC voltage signal at a second frequency, where the second frequency comprises an integral multiple of the first frequency. In some embodiments, the AC voltage assembly 140 may generate a third AC voltage signal at a third frequency, where the third frequency constitutes an integral multiple of the first frequency, different from the second frequency, and so forth. Thus, the second frequency, third frequency, etc., may be harmonics of the first frequency, where the frequency may be double, triple, etc., in comparison to the first frequency.

In the embodiment of FIG. 2, the AC voltage assembly 140 is shown to generate three different AC voltage signals, represented by V₁ cos(ωt+V₂ cos(2ωt+ϕ₂), and V₃ cos(3ωt+ϕ₃). For purposes of illustration, the AC voltage signals are shown as sinusoidal signals, while other waveform shapes are possible. The AC voltage assembly 140 may include a first AC voltage supply 142, second AC voltage supply 144, and third AC voltage supply 146, to generate a first AC voltage signal, second AC voltage signal, and third AC voltage signal, respectively. An AC voltage supply may be embodied using an RF amplifier driven by a synchronized signal generator. The general term V refers to the maximum amplitude of the AC voltage signal while the general term ϕ refers to the phase of the AC voltage signal. Thus, the maximum amplitude and the phase may differ among the different signals. In this embodiment, the second and third AC voltage signals represent a doubling and a tripling, respectively, of the frequency of the first signal co. As shown in FIG. 2, the AC voltage assembly 140 may include an adder 148, where the adder 148 sums the individual voltage signals and outputs a composite AC voltage signal 149 to the AC drift tube assembly 156.

In various embodiments, the composite AC voltage signal may be formed from AC voltage signals where the highest frequency of an AC voltage signal is approximately 120 MHz or less.

The composite AC voltage signal 149 is designed to adjust the phase dependence of ions processed by the AC drift tube assembly 156 in a manner that increases the acceptance at a downstream acceleration stage. In known linear accelerators of ion implantation systems, when a continuous ion beam is bunched for transmitting in packets to downstream acceleration stages, a certain fraction of the ion beam is lost to the walls or other surfaces due to the nature of the acceleration and bunching process. The acceptance refers to the percentage of ion beam (such as a percentage of beam current) not lost, and therefore accepted by the downstream acceleration stage. As noted, in known ion implantation apparatus employing linear accelerators, the acceptance may be on the order of 30% to 35% at a maximum, when various conditions are optimized. Such known ion implantation systems may drive a buncher with an AC voltage signal having a frequency of 10 MHz, 13.56 MHz, or 20 MHz, with a voltage amplitude in the range of tens of kV. Notably, the AC voltage signal in known ion implantation systems may be generated as a single frequency, simple AC voltage signal.

Notably, the fundamental component of the composite AC voltage signal may be simplified to V₁ cos(ωt), where the relative phase with respect to the other two AC voltage signals is given by the respective phase offsets, ϕ₂ or ϕ₃. As detailed below, these offsets may be adjusted to increase acceptance.

Turning to FIG. 3 there is shown the structure of an exemplary buncher of a linear accelerator, buncher 160, according to further embodiments of the disclosure. The buncher 160 may include a drift tube assembly 170, including a first grounded drift tube 182, arranged to accept a continuous ion beam, shown as accelerated ion beam 109. As shown, the first grounded drift tube 182 is connected to electrical ground. The drift tube assembly 170 may further include an AC drift tube assembly 180, arranged downstream to the first grounded drift tube 182. As discussed in detail below, the AC drift tube assembly 180, similarly to AC drift tube assembly 156, is arranged to receive an AC voltage signal(s), generally in the radio frequency range (RF range), which signal functions to accelerate and manipulate the accelerated ion beam 109. In the embodiment of FIG. 3, the AC drift tube assembly 180 includes three AC drift tubes, shown as AC drift tube 184, AC drift tube 186, and AC drift tube 188.

The drift tube assembly 170 further includes a second grounded drift tube 190, downstream to the AC drift tube assembly 180. As a whole, the drift tube assembly 170 is arranged as hollow cylinders to receive a continuous ion beam, conduct the ion beam through the hollow cylinders, and accelerate the ion beam in a manner that bunches the ion beam into discrete packets, shown as packet 109A, to be received and further accelerated by an acceleration stage 192, located downstream. As such the drift tube assembly 170 may constitute a multi-ring drift tube assembly having a length (along the direction of propagation of the ion beam) of at least 100 mm and less than 400 mm.

In the embodiment of FIG. 3, an AC voltage assembly 162 is provided, and arranged to send to the AC drift tube assembly 180, an AC voltage signal to drive a changing voltage at a powered drift tube of the AC drift tube assembly 180. The AC voltage assembly 162 may be configured wherein first AC voltage supply 142 drives the AC drift tube 184, the second AC voltage supply 144 drives the AC drift tube 186, and the third AC voltage supply 146 drives AC drift tube 188. These AC voltage signals may be synchronized in time by controller 164 to effectively generate a composite signal similar to composite AC voltage signal 149. While FIG. 3 illustrates a configuration where the lowest frequency AC voltage signal is supplied to the furthest upstream AC drift tube, in other embodiments, the lowest frequency AC voltage signal (V₁ cos(ωt+ϕ₀) may be applied to a different AC drift tube. The same applies to the intermediate frequency AC voltage signal (V₂ cos(2ωt+ϕ₂)), and the high frequency AC voltage signal (V₃ cos(3ωt+ϕ₃)). This configuration has the advantage over the configuration in FIG. 2 where the risk of a power supply interfering with other power supplies is avoided.

While the use of a multiple-frequency AC voltage signal to drive a buncher is possible, notably, using multiple frequencies to generate an AC voltage signal may entail more voltage supplies, and may lead to longer beamline, as detailed below. Accordingly, such a configuration in a beamline ion implanter has not heretofore been conceived. Notably, the present inventor has identified configurations where these considerations may be overcome by adjusting drive signals to markedly improve ion beam throughput, especially for ions having a mass in the range of common dopants such as boron, phosphorous, and the like. In particular, in the “single-ring” (where “ring” refers to an AC drift tube) buncher of FIG. 2 or the “triple-ring” buncher of FIG. 3, a composite AC voltage signal is produced, where the bunching of the ion beam is performed in a manner to improve phase coherence by using of an ion beam at a targeted distance from the AC drift tube assembly, and accordingly to increase the acceptance.

Turning to FIG. 4, there is shown a composite illustration including a depiction of the drift tube assembly 150, and a corresponding phase map, shown as a function of distance in millimeters along the beam path. The phase map is a graph illustrating the phase (shown on the right-hand ordinate) as a function of distance, with the position of the lone drift tube of the AC drift tube assembly 156, extending between 30 mm and 75 mm. At this location, the voltage (shown by left hand ordinate) applied to the AC drift tube assembly 156 reaches a maximum of approximately 18 kV, and is applied at a frequency of 40 MHz. The relative phase position of a series of 21 different rays of the accelerated ion beam 109 is shown to the right of the graph. The mass of the ions of the accelerated ion beam 109 is assumed to be 20 amu. As shown, the voltage reaches a maximum at the position of AC drift tube assembly 156, and is zero elsewhere. At the point of entry into the AC drift tube assembly 156 the 21 exemplary rays are equally spaced in phase at intervals of 18 degrees. When treated by the composite AC voltage signal given by V=V₁ cos(ωt+ϕ₁)+V₂ cos(2ωt+ϕ₂)+V₃ cos(3ωt+ϕ₃), such as generated by AC voltage assembly 140, the various rays converge in phase to the right, as shown.

At a location corresponding to 700 mm, 670 mm to the right of the entrance to the AC drift tube assembly 156, the phase difference between many of the rays is close to zero. Thus, when the entrance to an acceleration stage 158 is positioned at the 700 mm location, corresponding to a zero-phase difference between many of the rays, the acceptance may be a maximum. For an acceptance based upon +/−5-degree variation, in the example of FIG. 4, the acceptance at the accelerator is approximately 55%. In various other simulations, the maximum acceptance for the configuration of FIG. 4 has been calculated to be as high as 75%, a substantial improvement over the acceptance of 30%-35% in known ion implanters employing single frequency bunchers. For example, when V is set to equal 59.4 kV, the acceptance is 75%, while at 24 kV, the acceptance is 65%.

Notably, the same behavior for phase convergence shown in FIG. 4 using the illustration of the AC drift tube assembly 156, may be obtained by applying the same voltage parameters to the triple ring configuration of the AC drift tube assembly 180.

FIG. 5A and FIG. 5B are graphs illustrating the phase behavior of different rays of ion beams, highlighting the benefit of applying a composite AC voltage signal in accordance with the present embodiments. FIG. 5A continues with the composite AC voltage parameters of the embodiment of FIG. 4, while FIG. 5B illustrates an example of applying a simple AC voltage signal to the ion beam. In the illustration of FIG. 5B, the AC signal is given by V=V_(max) cos(ωt+ϕ), while in FIG. 5A the AC signal is given by V=V₁ cos(ωt+ϕ₁)+V₂ cos(2ωt+ϕ₂)+V₃ cos(3ωt+ϕ₃). The frequency ω is 40 MHz in both cases.

In the two different graphs, the phase behavior depicts the phase of given rays at a designated distance from a point near the entrance to the buncher as a function of the phase of the given rays at the entrance to the buncher. The designated distance is set at a distance where the phase of the different rays of the ion beam may be conveniently converged. Thus, with reference again to FIG. 4, in a packet 109A, operation of the AC drift tube assembly 156 tends to accelerate the phase-lagging ions 109A1 and tends to decelerate the phase-leading ions 109A2, leading to a phase convergence, such as at 700 mm.

In FIG. 5B, the most phase coherent condition, generating the highest relative acceptance of 35%, there is a small degree of phase difference at 400 mm even for differences in initial phase as small as 30 degrees. For other voltages the behavior is worse, as shown. Notably, the embodiment of FIG. 5A produces a convergence at 700 mm, somewhat longer than the single frequency buncher results, requiring a convergence at 400 mm. This result is in part due to the need to maintain AC voltage amplitude at a reasonable level for the composite AC voltage signal, such as approximately 20 kV. In the case of the single frequency buncher, operation at 20 kV AC voltage amplitude allows convergence at 400 mm. While the embodiment of FIG. 5A may entail a somewhat longer separation between the buncher and accelerator in comparison to the single frequency buncher architecture (700 mm vs 400 mm), a benefit is the substantially larger acceptance, and thus beam current, conducted into the main accelerator stages of a LINAC. In various additional embodiments, the convergence length may range from 300 mm to 1000 mm.

While additional stages of a LINAC may perform in a similar manner to the bunchers of the present embodiments, to accelerate and further bunch a packet of ions, these additional stages of the LINAC need not be driven by composite AC voltage signals as shown. In other words, because the composite AC voltage signal of the buncher has already mostly converged the phase of the various rays of the bunched ion beam at the entrance to an accelerator stage, further improvement of phase convergence may be less needed. This fact allows simpler design of AC voltage assemblies to drive the accelerator stages of the LINAC.

As an example, in one embodiment of a triple-frequency composite AC signal, the fundamental frequency for a first signal may be 40 MHz, while the first harmonic frequency may be 80 MHz for a second signal, added to the first signal, and a second harmonic frequency may be 120 MHz for a third signal, added to the first signal and the second signal.

Notably, while the above embodiments emphasize generating composite AC voltage signals based upon three AC voltage signals, and employing a multi-ring drift tube assembly including three drift tubes, in other embodiments, a composite AC voltage signal may be formed from two AC voltage signals, or four AC voltage signals. The embodiments are not limited in this context. Likewise, a multi-ring drift tube assembly according to other embodiments may employ two drift tubes or four drift tubes. The embodiments are not limited in this context.

FIG. 6 depicts an exemplary process flow 600 according to some embodiments of the disclosure. At block 602 an ion beam is generated as a continuous ion beam, such as by extraction from an ion source. As such, the ion beam may exhibit an ion energy in the range of several keV up to approximately 80 keV. Optionally, the continuous ion beam may be accelerated to generate an accelerated continuous ion beam. In one example, a DC accelerator column may be applied to accelerate the continuous ion beam. As such, the accelerated continuous ion beam may exhibit an ion energy of 200 keV to 500 keV or greater in some embodiments.

At block 604, the continuous ion beam is received in a multi-ring drift tube assembly. The multi-ring drift tube assembly may include a first grounded drift tube and a second grounded drift tube, as well as a multi-ring AC drift tube assembly, disposed between the first grounded drift tube and the second grounded drift tube.

At block 606, the continuous ion beam is conducted through a first AC drift tube of the multi-ring drift tube assembly while a first AC voltage signal is applied at a first frequency to the first AC drift tube.

At block 608, the continuous ion beam is conducted through a second AC drift tube of the multi-ring drift tube assembly while a second AC voltage signal is applied at a second frequency to the second AC drift tube. In various embodiments, the second frequency may be an integral multiple of the first frequency, such as double the first frequency. In an optional operation, the accelerated continuous ion beam may be conducted through a third AC drift tube of the multi-ring drift tube assembly while a third AC voltage signal is applied at a third frequency to the third AC drift tube. The third frequency may be an integral multiple of the first frequency, different from the second frequency. As such, the accelerated continuous ion beam may be output from the multi-ring drift tube assembly as a bunched ion beam.

In view of the foregoing, at least the following advantages are achieved by the embodiments disclosed herein. A first advantage is realized by providing a composite AC voltage signal to drive a buncher, so a substantially larger ion beam current may be transmitted through a LINAC disposed downstream. A further advantage is the ability to drive a given AC signal from a given power supply of a plurality of AC power supplies to a dedicated electrode, avoiding interference between power supplies that may occur when coupled to a common multiple power supplies are coupled to drive multiple AC voltage signals through a common electrode. Another advantage realized by using a DC acceleration column is the reduced space charge effects and better efficiency of buncher designed to generate a high frequency composite AC voltage signal to drive the buncher.

While certain embodiments of the disclosure have been described herein, the disclosure is not limited thereto, as the disclosure is as broad in scope as the art will allow and the specification may be read likewise. Therefore, the above description are not to be construed as limiting. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto. 

The invention claimed is:
 1. An apparatus, comprising: a multi-ring drift tube assembly, the multi-ring drift tube assembly comprising: a first grounded drift tube, arranged to accept a continuous ion beam; first, second, and third AC drift tubes, arranged in series, downstream to the first grounded drift tube; and a second grounded drift tube, downstream to the first, second, and third AC drift tubes; an AC voltage assembly, electrically coupled to the first, second, and third AC drift tubes, the AC voltage assembly comprising: a first AC voltage source, coupled to deliver a first AC voltage signal at a first frequency to a first AC drift tube of at least two AC drift tubes; a second AC voltage source, coupled to deliver a second AC voltage signal at a second frequency to a second AC drift tube of the at least two AC drift tubes, wherein the second frequency comprises an integral multiple of the first frequency; and a third AC voltage source, coupled to deliver a third AC voltage signal at a third frequency to the third AC drift tube, wherein the third frequency comprises an integral multiple of the first frequency, different from the second frequency.
 2. The apparatus of claim 1, wherein the first frequency comprises a frequency of at least 20 MHz.
 3. The apparatus of claim 1, wherein the first frequency comprises a frequency of at least 20 MHz, wherein the second frequency comprises a multiple of two of the first frequency, and wherein the third frequency comprises a multiple of three of the first frequency.
 4. The apparatus of claim 3, wherein the first frequency comprises a frequency of at least 40 MHz.
 5. The apparatus of claim 1, wherein the multi-ring drift tube assembly is arranged to output the continuous ion beam as a bunched ion beam.
 6. The apparatus of claim 1, wherein the multi-ring drift tube assembly is arranged to generate an acceptance in an accelerator stage, downstream to the multi-ring drift tube assembly, of at least 50%, wherein less than 50% of ion of the ion beam is lost.
 7. The apparatus of claim 1, wherein an amplitude of the first AC voltage signal and the second AC voltage signal is less than 25 kV.
 8. The apparatus of claim 1, wherein a length of the multi-ring drift tube assembly is at least 100 mm and less than 400 mm.
 9. The apparatus of claim 1, wherein the continuous ion beam comprises an ion energy of at least 250 kV.
 10. An ion implantation system, comprising: an ion source to generate a continuous ion beam; a drift tube assembly, disposed downstream of the ion source, the drift tube assembly comprising: a first grounded drift tube, arranged to accept the continuous ion beam; an AC drift tube assembly, arranged downstream to the first grounded drift tube; and a second grounded drift tube, downstream to the AC drift tube assembly; and an AC voltage assembly, arranged to send to the AC drift tube assembly a first AC voltage signal at a first frequency, and a second AC voltage signal at a second frequency, wherein the second frequency comprises an integral multiple of the first frequency, and wherein the second frequency is twice the first frequency, wherein the AC voltage assembly is arranged to send a third AC voltage signal at a third frequency to the AC drift tube assembly, the third frequency being three times the first frequency.
 11. The ion implantation system of claim 10, wherein the drift tube assembly is configured to output the continuous ion beam as a bunched ion beam.
 12. The ion implantation system of claim 10, further comprising an adder, coupled to receive the first AC voltage signal, the second AC voltage signal, and the third AC voltage signal, and to output a composite AC voltage signal comprising a sum of the first AC voltage signal, the second AC voltage signal, and the third AC voltage signal, wherein the AC drift tube assembly comprises a first AC drift tube, arranged to receive the composite AC voltage signal.
 13. The ion implantation system of claim 10, wherein the AC drift tube assembly comprises a first AC drift tube, a second AC drift tube, and a third AC drift tube, arranged in series with the first AC drift tube and the second AC drift tube, and wherein the AC voltage assembly comprises a first AC voltage source, coupled to deliver the first AC voltage signal to the first AC drift tube, a second AC voltage source, coupled to deliver the second AC voltage signal to the second AC drift tube, and a third AC voltage source, coupled to deliver a third AC voltage signal at a third frequency to the third AC drift tube, wherein the third frequency comprises an integral multiple of the first frequency, different from the second frequency.
 14. The ion implantation system of claim 13, wherein the first frequency comprises a frequency of at least 20 MHz, and wherein the third frequency comprises a frequency of 120 MHz or less.
 15. A method of treating an ion beam, comprising: generating the ion beam as a continuous ion beam; receiving the continuous ion beam in a multi-ring drift tube assembly; conducting the ion beam through a first AC drift tube of the multi-ring drift tube assembly while applying a first AC voltage signal at a first frequency to the first AC drift tube; and conducting the ion beam through a second AC drift tube of the multi-ring drift tube assembly, downstream of the first AC drift tube while applying a second AC voltage signal at a second frequency to the second AC drift tube, the second frequency comprising an integral multiple of the first frequency, wherein the ion beam is output from the multi-ring drift tube assembly as a bunched ion beam, wherein the continuous ion beam comprises an ion energy of at least 250 keV.
 16. The method of claim 15, further comprising receiving the bunched ion beam in a first stage of a linear accelerator, wherein an acceptance of the ion beam is at least 50%.
 17. The method of claim 15, further comprising conducting the ion beam through a third AC drift tube of the multi-ring drift tube assembly, downstream of the second AC drift tube while applying a third AC voltage signal at a third frequency to the third AC drift tube, the third frequency comprising an integral multiple of the first frequency, different from the first frequency. 